Method of producing strengthened alloy

ABSTRACT

To provide a method of producing a strengthened alloy capable of shortening the time necessary for aging treatment and obtaining a strengthened alloy with enhanced tensile strength. The method of producing a strengthened alloy comprises a solution treatment step S 1  of immersing an alloy material into molten lithium held at a solution treatment temperature higher than solution temperature of solute metal of the alloy material, a solution stop step S 2  of immersing the alloy material into molten lithium held at a cooling temperature lower than the solution treatment temperature after the solution treatment step S 1 , an aging treatment step S 3  of immersing the alloy material into molten lithium held at an aging treatment temperature lower than the solution temperature after the solution stop step S 2 , and an aging stop step S 4  of immersing the alloy material into molten lithium held at an aging stop temperature lower than the aging treatment temperature after the aging treatment step S 3.

This application is based on and claims the benefit of priority fromJapanese Patent Application No. 2009-103733, filed on 22 Apr. 2009, thecontent of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing a strengthenedalloy.

2. Related Art

Intended mechanical properties of carbon steels and other alloys havebeen heretofore obtained by solution treatments such as quenching andthen aging treatments such as tempering.

Among others, titanium alloys are lightweight, highly strong, highlyheat-resistant, and highly corrosion-resistant, therefore, applicationsthereof have been rapidly growing in recent years. The basic crystalstructure of metal titanium is close-packed hexagonal (α-phase) at roomtemperature; however, body-centered cubic crystal (β-phase) is formed athigher temperatures than the β-transformation temperature (about 885° C.in pure titanium) and α-phase is formed again when being cooled further.With regard to titanium alloys containing various alloy elements such asaluminum (Al) and vanadium (V) in metal titanium, various metalstructures are formed by changing their constituent ratios, heatingtemperature, heating rate, or cooling rate, and titanium alloys exhibitvarious charcteristics after heating.

When an α+β type alloy among titanium alloys, in which α-phase andβ-phase coexist at ambient temperature, undergoes a solution treatmentby heating to a temperature that is higher than that at which alloyelements disperse into solute metal to form a solid solution (solutiontemperature), a β-phase is formed in the metal structure and thenα′-martensite phase is formed inside the β-phase upon rapid cooling.When the solution-treated alloy is further heated at a temperature thatis less than the solution temperature (aging treatment), a fine α-phaseis formed inside the β-phase. Structural ratio of the phases formed inthe metal structure and crystal size of the phases change in this stage;therefore, alloys are formed that have a strength or elongationdifferent from that before the heat treatment.

Here, means is publicly known in which a solution-treated titanium alloyis heated to a predetermined temperature in an induction heatingapparatus and aging-treated by holding the titanium alloy at a constanttemperature for a predetermined period followed by air cooling (seeNon-Patent Documents 1 and 2). Furthermore, means is publicly known inwhich a solution-treated titanium alloy is aging-treated by immersinginto molten lithium at a predetermined temperature followed by naturalcooling (see Patent Document 1).

Japanese Unexamined Patent Application, First Publication No.2006-016691

Tatsuro Morita and three researchers, “Strengthening of Ti-6Al-4V alloyby Short-Time 2-Stage Induction Heat Treatment”, J. Japan Inst. Metals,The Japan Institute of Metals, October 2002, Vol. 68, No. 10, pp.862-867.

Tatsuro Morita and three researchers, “Influence of Short-Time DuplexHeat Treatment on Fatigue Strength of Ti-6Al-4V Alloy”, Journal of theSociety of Materials Science, Japan, The Society of Materials Science,Japan, April 2007, Vol. 56, No. 4, pp. 345-351.

SUMMARY OF THE INVENTION

However, when titanium alloy is heat-treated by rapidly heating to apredetermined temperature (e.g. by 8 seconds) in an induction heatingapparatus as described in Non-Patent Document 1, the surface temperatureof the induction-heated titanium alloy is unlikely uniform and tends tobe heated higher than the predetermined temperature due to overshoot ofthe heating apparatus. Therefore, in Non-Patent Document 2 publishedlater, the induction heating is improved so that the time required toreach the predetermined temperature is longer (e.g. by 360 seconds) whenthe heat treatment is carried out in an induction heating apparatus,this however requires a longer time for the heat treatment. Furthermore,although the alloys obtained in accordance with the means of Non-PatentDocuments 1 and 2 show an increase in 0.2% proof stress and tensilestrength, ductility tends to decrease against an increase in strength.

Another means may be exemplified in which heat treatment is carried outby immersing a solution-treated titanium alloy into molten lithiumfollowed by natural cooling as described in Patent Document 1. As aresult of treatment of titanium alloy using the means of the presentinventors, all of 0.2% proof stress, tensile strength, and ductilitywere enhanced, but these 0.2% proof stress, tensile strength, andductility are desired to be enhanced further.

Accordingly, it is an object of the present invention to provide amethod of producing a strengthened alloy that can shorten the timenecessary for heat treatment and obtain the strengthened alloy of which0.2% proof stress, tensile strength, and ductility are enhanced further.

The present inventors have discovered that when a solution-treated alloymaterial undergoes an aging treatment step of immersing into moltenlithium held at an aging treatment temperature and then an aging stopstep of immersing into molten lithium held at an aging stop temperaturelower than the aging treatment temperature, excessive growth of α-phasewith larger tensile strength is suppressed and transformation of β-phasewith larger ductility, formed between α-phases, into α-phase issuppressed, thereby achieving the present invention.

In a first aspect of the present invention, a method of producing astrengthened alloy, the method comprising: a solution treatment step ofimmersing an alloy material into molten lithium held at a solutiontreatment temperature higher than solution temperature of solute metalof the alloy material; a solution stop step of immersing, after thesolution treatment step, the alloy material into molten lithium held ata cooling temperature lower than the solution treatment temperature; anaging treatment step of immersing, after the solution stop step, thealloy material into molten lithium held at an aging treatmenttemperature lower than the solution temperature; and an aging stop stepof immersing, after the aging treatment step, the alloy material intomolten lithium held at an aging stop temperature lower than the agingtreatment temperature.

In a second aspect of the method according to the first aspect of thepresent invention, the alloy material is immersed into molten lithium atnot more than 350° C. in the aging stop step.

In a third aspect of the method according to the first or second aspectof the present invention, a titanium alloy is used as the alloymaterial.

According to the present invention, when a solution-treated alloymaterial undergoes an aging treatment step of immersing into moltenlithium held at an aging treatment temperature and then an aging stopstep of immersing into molten lithium held at an aging stop temperaturelower than the aging treatment temperature, excessive growth of α-phasewith larger tensile strength is suppressed and transformation of β-phasewith larger ductility, formed between α-phases, into α-phase issuppressed. Therefore, the method of producing a strengthened alloy,that can shorten the time necessary for heat treatment and obtain thestrengthened alloy of which 0.2% proof stress, tensile strength, andductility are enhanced further, can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing an embodiment of the method of producinga strengthened alloy according to the present invention;

FIG. 2 is a graph showing a change in surface temperature of an alloymaterial in the method of producing a strengthened alloy according tothe present invention; and

FIG. 3 is a cross-sectional view showing a production device preferablyused in the method of producing a strengthened alloy according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the method of producing a strengthened alloy accordingto the present invention is explained with reference to figures. FIG. 1is a flow chart showing an embodiment of the method of producing astrengthened alloy according to the present invention. FIG. 2 is a graphshowing a change in surface temperature of an alloy material in themethod of producing a strengthened alloy according to the presentinvention. FIG. 3 is a cross-sectional view showing a production devicepreferably used in the method of producing a strengthened alloyaccording to the present invention.

As shown in FIG. 1, the method of producing a strengthened alloy of thisembodiment includes a solution treatment step S1, a solution stop stepS2, an aging treatment step S3, and an aging stop step S4.

Alloy Material

The alloy material 50 used in this production method is an alloy ofwhich the solution temperature is higher than the melting point (181°C.) of molten lithium and lower than the boiling point (1342° C.) ofmolten lithium among the materials of which the tensile strength ofα-phase is larger than that of β-phase and the breaking elongation ofβ-phase is larger than that of α-phase. The alloy material 50 satisfyingthese requirements may be exemplified by titanium alloy. Morespecifically, Ti-6Al-4V alloy may be exemplified. When the Ti-6Al-4Valloy is used, a part of metal structure thereof transforms the phasefrom α-phase of hexagonal close-packed crystal to β-phase ofbody-centered cubic crystal having a large breaking elongation by thesolution treatment step S1 to heat rapidly the surface temperature ofthe alloy material to a solution treatment temperature higher than 850°C.

The alloy material 50 may be shaped by rolling or cutting work prior tothe heat treatment. Thereby, processing strains accumulate inside thealloy material 50, therefore, crystal nuclei can easily form when thealloy material 50 undergoes heating and cooling and a finer metalstructure can be achieved after the heat treatment.

Heat Treatment Device

The heat treatment device used in the production method may beexemplified by the heat treatment device 1 equipped with a solutiontreatment bath 12, a solution stop bath 13, an aging heating bath 14,and an aging stop bath 15 which are inside a heat treatment room 11 asshown in FIG. 3, for example. The heat treatment device 1 is equippedwith a transport mechanism 27 to move and sequentially immerse the alloymaterial 50 into the molten lithium L1 to L4 respectively contained inthe solution treatment bath 12, the solution stop bath 13, the agingheating bath 14, and the aging stop bath 15.

Here, the heat treatment device 1 may be provided with a sealableentrance room 30 at one side of the heat treatment room 11 and asealable exit room 40 at another side. In this stage, the entrance room30 is provided with a door 31 to separate the inside of the entranceroom 30 and ambient air, and a door 32 to separate the inside of theentrance room 30 and the inside of the heat treatment room 11. On theother hand, the exit room 40 is provided with a door 41 to separate theinside of the exit room 40 and the inside of the heat treatment room 11,and a door 42 to separate the inside of the exit room 40 and ambientair. The heat treatment room 11 is provided with the entrance room 30and the exit room 40 and an inactive gas such as argon gas non-reactivewith the molten lithium L1 to L4 is filled inside the heat treatmentroom 11, thereby the inactive gas is unlikely to leak outside the heattreatment room 11, therefore, reaction between the molten lithium andair or moisture can be inhibited.

(S1) Solution Treatment Step

The alloy material 50 is heated and the surface temperature of the alloymaterial 50 is held at a solution treatment temperature T1 higher thanthe solution temperature of the alloy material 50. Thereby, a part ofalloy elements of the alloy material 50 disperses into solute metal toform a solid solution of β-phase of the solute metal.

A means to immerse the alloy material 50 into the molten lithium L1,heated at the solution treatment temperature T1, is used as the means toheat the alloy material 50. Thereby, temperature overshoot of the alloymaterial 50 is avoided. Consequently, the decrease of tensile strength,breaking elongation, and 0.2% proof stress of the alloy material 50 dueto abnormal heating of the alloy material 50 can be suppressed.Furthermore, heating can be rapidly conducted to the alloy material 50through the molten lithium with a higher thermal conductivity,therefore, the alloy material 50 can be heated more promptly and thesolid solution of β-phase of the solute metal can be formed quickly.Therefore, inclusion of hydrogen and/or oxygen into the metal structurecan be reduced during the solution treatment of the alloy material 50while avoiding the growth and enlargement of crystals in the metalstructure, thus the decrease of tensile strength, breaking elongation,and 0.2% proof stress due to crystal enlargement or void formation inthe metal structure can be suppressed.

The heating means may be specifically exemplified by the means ofcontaining the molten lithium L1, heated at the solution treatmenttemperature T1, in the solution treatment bath 12 and immersing thealloy material 50, conveyed from the entrance room 30 to the heattreatment room 11 of the heat treatment device 1, into the moltenlithium L1, as shown in FIG. 3. Here, a heater 21 to heat the moltenlithium L1 is used to control the temperature of the molten lithium L1contained in the solution treatment bath 12, for example.

The solution treatment temperature T1 in the solution treatment step S1is higher than the solution temperature of the alloy material 50. Here,the solution temperature of the alloy material 50, which depends oncomposition etc. of the alloy material 50, is often in the range of 700°C. to 1100° C., for example. More specifically, the solution temperaturefor the Ti-6Al-4V alloy is in the range of 850° C. to 1000° C.

The solving time, to hold the surface temperature of the alloy material50 heated in the solution treatment step S1 at the solution treatmenttemperature T1, is appropriately set depending on the temperature of theheated alloy material 50, thickness of the alloy material 50, andapplication of the strengthened alloy, and the time may be 10 to 300seconds. Particularly, when the solving time is at least 10 seconds, thetemperature of the inner portion of the alloy material 50 can besufficiently increased before the end of the solving time, therefore,the solid solution of β-phase of the solute metal and alloy constituentscan also be formed in the inner portion of the alloy material 50. On theother hand, when the solving time is not more than 300 seconds, growthand enlargement of β-crystal formed by heating can be suppressed,therefore, miniaturization of the metal structure of the alloy material50 can be promoted. Here, the means to hold the surface temperature ofthe alloy material 50 may be exemplified by holding the temperature ofmolten lithium L1, into which the alloy material 50 is immersed, at thesolution treatment temperature T1 by adjusting with the heater 21, forexample.

(S2) Solution Stop Step

After the solution treatment step S1, the alloy material 50 is cooled tothe solution stop temperature T2 that is lower than the solutiontemperature (step S21 of FIG. 2). The phase transition from β-phase toα-phase is suppressed by cooling the alloy material 50 to the solutionstop temperature T2 and thus a part of β-phase remains in the metalstructure under a condition of solid solution with alloy constituents.When the alloy material 50 is rapidly cooled at this stage,α′-martensite phase is formed in the metal structure of the alloymaterial 50. Accordingly, a microstructure of solid solution of alloyelements consisting of the α′-martensite phase and the remaining β-phasecan be formed in the alloy material 50 by rapidly cooling to thesolution stop temperature T2. The solution stop temperature T2, which isappropriately selected depending on the composition of the alloymaterial 50, is preferably lower than 500° C. and more preferably lowerthan 300° C. when the Ti-6Al-4V alloy is used as the alloy material 50,for example.

A means of immersing the alloy material 50 into the molten lithium L2held at the solution stop temperature T2 is used as the means of rapidlycooling the alloy material 50 to the solution stop temperature T2.Thereby, heat is rapidly removed from the alloy material 50 by themolten lithium L2 with a higher thermal conductivity, thus the alloymaterial 50 can be cooled rapidly still further.

The cooling means may be specifically exemplified by the means ofimmersing the alloy material 50 into the molten lithium L2, held at acooling temperature (solution stop temperature T2) and contained in thesolution stop bath 13, within the heat treatment room 11 of the heattreatment device 1, as shown in FIG. 3. Here, a heater 22 to heat themolten lithium L2 and a fan 23 to cool the molten lithium L2 are used inorder to control the temperature of the molten lithium L2 contained inthe solution stop bath 13, for example.

After cooling the alloy material 50 by immersing into the molten lithiumL2 of the solution stop bath 13, the alloy material 50 may be furthercooled to a temperature lower than the melting point of lithium (181°C.), more specifically to room temperature (ambient temperature) (stepS22 of FIG. 2). Thereby, deposition of α-phase inside the alloy material50 is reduced further, therefore, decrease of α′-martensite phase andthe remaining β-phase due to the passage of time can be suppressed. Inaddition, the cooling means for the alloy material 50 after removal fromthe solution stop bath 13 may be exemplified by natural cooling under aninert gas atmosphere within the heat treatment room 11 of the heattreatment device 1 as shown in FIG. 3, for example.

Here, after the solution stop step S2 the alloy material 50 may undergomechanical processing such as rolling. Thereby, processing strains areformed in the internal structure of the alloy material 50. Therefore,when it is heated again, recrystallization of internal structure canprogress simultaneously from these processing strains, andminiaturization of the crystals formed during re-heating can bedesigned.

(S3) Aging Treatment Step

The alloy material 50 is heated to the aging treatment temperature T3.In this stage, the alloy material 50 includes a solid solution in whichalloy elements dissolve into the solute metals of α-phase, α′-martensitephase, and β-phase. The β-phase including the alloy elements tends to bedecomposed into compounds including fine α-phase and alloy elements byheating the alloy material 50 to the aging treatment temperature T3.Therefore, with respect to the strengthened alloy obtained after theaging treatment of the alloy material 50, the ratio of the α-phasecrystal can be increased, and the tensile strength and 0.2% proof stressof the alloy material 50 can be increased.

Here, the aging treatment temperature T3 during the aging treatment stepS3 is lower than the β-transformation temperature at which solute metaltransforms to β-phase (Ti-6Al-4V alloy: 885° C.) and is preferably inthe range of 400° C. to 650° C. Particularly, when the Ti-6Al-4V alloyis used as the alloy material 50, it is more preferable that the surfacetemperature of the alloy material 50 during the aging treatment step S3is maintained at not more than 600° C. Because when the temperature ofthe alloy material 50 becomes higher than the temperature rangementioned above, the α-phase formed in the alloy material 50 is likelyto transform into β-phase with lower tensile strength, and eventually,tensile strength and 0.2% proof stress at the portions suddenlydecrease.

The heating of the alloy material 50 to the aging treatment temperatureT3 is carried out for a short time in the aging treatment step S3.Thereby, a part of β-phase of the solute metal decomposes to formα-phase with larger tensile strength and lower particle diameter; on theother hand, compounds are produced between the solute metal and thealloy elements and β-phase with high ductility remains between theseproducts. Therefore, with respect to the strengthened alloy after theaging treatment step S3, the tensile strength and/or 0.2% proof stressand also breaking elongation can be increased. Accordingly, strengthenedalloy having hardness as well as toughness is obtainable.

The means of heating the alloy material 50 to the aging treatmenttemperature T3 for a short time may be exemplified by the means ofimmersing the alloy material 50 into molten-state lithium (moltenlithium L3) heated to the aging treatment temperature T3. Thereby, heatis rapidly conducted to the alloy material 50 by molten lithium with athermal conductivity (41.4 W/mK) higher than that of air (2.41×10⁻²W/mK). Therefore, a great number of α-phase crystal nuclei can depositfrom β-phase of the solute metal within a short time and α-phase finetexture of the solute metal can be formed in these crystal nuclei.Accordingly, the particle diameter of α-phase crystals contained in thestrengthened alloy can be lowered, and the breaking elongation can beincreased by allowing the β-phase crystals to form in the α-phasecrystals. In this stage, the surface temperature of the alloy material50 becomes approximately the same as the temperature of the moltenlithium L3, for about 1 to 2 seconds after immersing the alloy material50 into the molten lithium L3.

The heating means may be specifically exemplified by the means ofimmersing the alloy material 50 into the molten lithium L3 heated to theaging treatment temperature T3 in the aging heating bath 14 which iscontained in the heat treatment room 11 of the heat treatment device 1shown in FIG. 3. A heater 24 to heat liquid lithium L3 is used tocontrol the temperature of the molten lithium L3 in the aging heatingbath 14, for example.

In the aging treatment step S3, the aging treatment time of immersingthe alloy material 50 into the molten lithium L3 heated to the agingtreatment temperature T3, to be decided depending on the size and/orshape of the alloy material 50, is preferably 30 seconds to 30 minutes,for example. The inner portion of the alloy material 50 can also beeasily heated to the aging treatment temperature T3 for not less than 30minutes, and therefore, the tensile strength and/or 0.2% proof stress ofthe alloy material 50 can be enhanced further. On the other hand,α-phase of solute metal of the alloy material 50 is unlikely to grow andenlarge unnecessarily when the aging treatment time is not more than 30minutes, and the lowering of breaking elongation due to decease ofβ-phase of solute metal can be suppressed.

(S4) Aging Stop Step

After the aging treatment step S3 the alloy material 50 is cooled downfor a short time until the surface temperature reaches the aging stoptemperature T4 (step S41 of FIG. 2). Thereby, the α-phase crystal ofsolute metal formed in the aging treatment step S3 is suppressed fromadditional growth on the surface of the alloy material 50, andtransforming from β-phase remaining in the alloy material 50 intoα-phase is suppressed. Furthermore, the alloy metal is unlikely tobecome supersaturated within the solute metal due to excessive formationof α-phase and thus compounds of alloy metal are unlikely formed.Therefore, the average particle diameter of α-phase of the strengthenedalloy can be lowered after the aging stop, and the breaking elongationof the resulting strengthened alloy can be increased while maintainingthe fine metal structure consisting of α-phase crystal and remainingβ-phase. The fact that 0.2% proof stress, tensile strength, andductility of strengthened alloy can be increased by the cooling afterthe aging treatment step S3 is newly found.

A means of immersing the alloy material 50 into the molten lithium L4held at the aging stop temperature T4 is used as the means of coolingthe alloy material 50 for a short time. Thereby, heat is rapidly removedfrom the alloy material 50 by molten lithium with a thermal conductivity(41.4 W/mK) higher than that of air (2.41×10⁻² W/mK). Therefore, thealloy material 50 can be rapidly cooled. Here, the aging stoptemperature T4 is set within a temperature range of higher than themelting point of lithium (181° C.) and lower than the aging treatmenttemperature T3; for example, when the temperature is set at not morethan 350° C., preferably at not more than 300° C., the growth of α-phaseof the solute elements can be suppressed. In addition, the aging stoptemperature T4 may be the same as the solution stop temperature T2 inthe solution stop step S2.

The cooling means may be specifically exemplified by the means ofimmersing the alloy material 50 into the aging stop bath 15 containingthe molten lithium L4 in the heat treatment room 11 of the heattreatment device 1 as shown in FIG. 3. Here, a heater 25 to heat themolten lithium L4 and a fan 26 to cool the molten lithium L4 are used inorder to control the temperature of the molten lithium L4 contained inthe aging stop bath 15. In this stage, the surface temperature of thealloy material 50 immersed into molten lithium L4 is cooled down to theaging stop temperature T4 in about 1 to 2 seconds.

In the aging stop step S4, the time that the alloy material 50 isimmersed into the molten lithium L4 held at the aging stop temperatureT4, is chosen depending on the size and/or shape of the alloy material50, and is preferably not less than 10 seconds. Particularly, when theimmersing time is not less than 10 seconds, the growth of α-phasecrystal can be suppressed in the inner portion of the alloy material 50,therefore, the tensile strength and/or 0.2% proof stress of the alloymaterial 50 can be enhanced further.

After cooling down the alloy material 50 by immersing into the moltenlithium L4 of the aging stop bath 15, the alloy material 50 is furthercooled to a temperature lower than the melting point of lithium (181°C.), specifically to room temperature (ambient temperature) (step S42 ofFIG. 2). In addition, the cooling means for the alloy material 50 afterremoval from the aging stop bath 15 may be exemplified by naturalcooling under an inert gas atmosphere inside the heat treatment room 11of the heat treatment device 1 as shown in FIG. 3, for example.

Lithium attached to the surface of the strengthened alloy obtained bythe aging stop step S4 is removed with a washing means. Thereby, heatgeneration etc. due to contact of the lithium with air and/or water canbe reduced. The washing means for the strengthened alloy may beexemplified by a means of immersing into a large amount of water andultrasonically cleaning.

As described above, the method of producing a strengthened alloyaccording to the present invention is explained by one embodiment;however, the present invention should not be limited to the embodiment.

For example, the heat treatment device 1 is not limited to thisembodiment, the solution stop bath 13 and the aging stop bath 15 may beconstructed as an identical molten lithium bath and the solutiontreatment bath 12 and the aging heating bath 14 may be constructed as anidentical molten lithium bath. Thereby, the number of molten lithiumbaths used for producing the strengthened alloy is decreased, therefore,the heat treatment device 1 can be simplified and the amount of lithiumused for the heat treatment device 1 can be reduced. In this regard, ifthe solution treatment bath 12 and the aging heating bath 14 areconstructed as an identical molten lithium bath, the temperature ofmolten lithium is set to the solution treatment temperature T1 when usedas the solution treatment bath 12 and the temperature of molten lithiumis set to the aging treatment temperature T3 when used as the agingheating bath 14 by way of controlling the output of the heater 21 (24).

EXAMPLES

The present invention is explained more specifically with reference toexamples hereinafter; however, the present invention is not limited tothe examples.

Example 1

Ti-6Al-4V alloy (β-transformation temperature: 885° C.) was used as thealloy material. A thermocouple was attached to the surface of the alloymaterial in order to measure the surface temperature.

A heat treatment device, having a heat treatment room sealed and filledwith argon gas, was used in order to heat and cool the alloy material.An entrance room sealable by a door was provided at one side of the heattreatment room and an exit room sealable by a door was provided atanother side. Furthermore, a solution treatment bath, a solution stopbath, an aging heating bath, and an aging stop bath containingrespectively molten lithium were provided inside the heat treatmentroom.

Among these, the temperature of molten lithium contained in the solutiontreatment bath was fixed at the solution treatment temperature of 980°C. using a heater, and the alloy material, conveyed from the entranceroom of the heat treatment device to the heat treatment room, wasimmersed into the molten lithium of the solution treatment bath.Thereafter, the temperature of molten lithium immersing the alloymaterial was held for a solving time of 60 seconds.

Then the alloy material was transferred into the solution stop bath andthe alloy material was cooled rapidly. When the alloy material wasimmersed into the solution stop bath, the temperature of molten lithiumcontained in the solution stop bath was controlled to a solution stoptemperature of 200° C. using a heater to heat the molten lithium and afan to cool the molten lithium. After immersing into the molten lithiumof the solution stop bath, the alloy material was further cooled toambient temperature by exposing to an inert gas atmosphere in the heattreatment room.

Then the alloy material was immersed into the aging heating bath. Whenthe alloy material was immersed into the aging heating bath, thetemperature of molten lithium contained in the aging heating bath wascontrolled to an aging treatment temperature of 550° C. using a heater.After immersing the alloy material into the aging heating bath, thetemperature of molten lithium immersing the alloy material was held foran aging treatment time of 10 minutes to hold the surface temperature ofthe alloy material at the aging treatment temperature.

Then the alloy material was transferred to the aging stop bath to coolthe alloy material rapidly. When the alloy material was immersed intothe aging stop bath, the temperature of molten lithium contained in theaging stop bath was controlled to an aging stop temperature of 200° C.using a heater and a fan. In this stage, the surface temperature of thealloy material was lowered to the aging treatment temperature in 2seconds. After immersing into the molten lithium of the aging stop bath,the alloy material was further cooled to ambient temperature by exposingto an inert gas atmosphere in the heat treatment room.

The strengthened alloy after heat treatment was ultrasonically cleanedwith a large amount of water to remove the lithium attached to thesurface and then tensile test in accordance with JIS Z 2241 was carriedout to measure 0.2% proof stress, tensile strength, and breakingelongation.

Comparative Example 1

Comparing to Example 1, the alloy material was cooled without immersinginto molten lithium of the aging stop bath in the aging stop step S4.That is, the alloy material taken out of the aging treatment bathsimilarly as Example 1 was cooled to ambient temperature by exposing toan inert gas atmosphere in the heat treatment room without immersinginto the aging stop bath. In this stage, the time when the surfacetemperature of the alloy material became lower than 200° C., same as theaging stop temperature, was one hour after removing the alloy materialfrom the molten lithium. The others were similar as those of Example 1.

Comparative Example 2

Comparing to Example 1, the aging treatment time was prolonged in theaging treatment step S3 and the alloy material was cooled withoutimmersing into molten lithium of the aging stop bath in the aging stopstep S4. That is, the alloy material taken out of the solution treatmentbath and cooled to ambient temperature similarly as Example 1 was heatedto an aging treatment temperature of 550° C. using a heater withoutimmersing into the aging heating bath. In this stage, the alloy materialwas heated to the aging treatment temperature in about 2 seconds. Afterthe surface temperature of the alloy material had reached the agingtreatment temperature, the surface temperature of the alloy material washeld at the aging treatment temperature for 2 hours.

Then heating of the alloy material by a heater was stopped, and thealloy material was exposed to an inert gas atmosphere in the heattreatment room to cool to ambient temperature. In this stage, the timethat the surface temperature of the alloy material became lower than200° C., same as the aging stop temperature, was one hour after removingthe alloy material from the molten lithium. The others were similar asthose of Example 1.

Measured values of mechanical strength based on tensile test inaccordance with JIS Z 2241 are shown in Table 1 below with respect tothe alloy materials of Example 1, Comparative Examples 1 and 2 of beforethe solution treatment step S1 (before test) and after the aging stopstep S4 (after test).

TABLE 1 0.2% proof Breaking stress Tensile strength elongation (N/mm²)(N/mm²) (%) Example 1 Before test 879 991 16.3 After test 1098 1266 19.1Difference +219 +275 +2.8 Comparative Before test 879 991 16.3 Example 1After test 1051 1213 17.5 Difference +172 +222 +1.2 Comparative Beforetest 1003 1022 15.7 Example 2 After test 1322 1429 4.3 Difference +319+407 −11.4

From the results of Example and Comparative Examples described above,the following can be verified.

With regard to Example 1 in which the alloy material was immersed intomolten lithium at the aging stop temperature, the cooling time to theaging stop temperature was shortened and also all of 0.2% proof stress,tensile strength, and breaking elongation of the aging strengthenedalloy were enhanced, compared to Comparative Examples 1 and 2 in whichthe alloy material was not immersed into molten lithium at the agingstop temperature.

1. A method of producing a strengthened alloy, the method comprising: asolution treatment step of immersing an alloy material into moltenlithium held at a solution treatment temperature higher than solutiontemperature of solute metal of the alloy material; a solution stop stepof immersing, after the solution treatment step, the alloy material intomolten lithium held at a cooling temperature lower than the solutiontreatment temperature; an aging treatment step of immersing, after thesolution stop step, the alloy material into molten lithium held at anaging treatment temperature lower than the solution temperature; and anaging stop step of immersing, after the aging treatment step, the alloymaterial into molten lithium held at an aging stop temperature lowerthan the aging treatment temperature.
 2. The method according to claim1, wherein the alloy material is immersed into molten lithium at notmore than 350° C. in the aging stop step.
 3. The method according toclaim 1, wherein a titanium alloy is used as the alloy material.